fix: add db.util.make_topo_sortkey() function

src/wuttjamaican/db/util.py

@@ -27,6 +27,7 @@ Database Utilities
 import uuid as _uuid
 
 import sqlalchemy as sa
+from sqlalchemy import orm
 from sqlalchemy.dialects.postgresql import UUID as PGUUID
 
 from wuttjamaican.util import make_uuid
@@ -127,3 +128,26 @@ def uuid_fk_column(target_column, *args, **kwargs):
     if not args:
         args = (sa.String(length=32), sa.ForeignKey(target_column))
     return sa.Column(*args, **kwargs)
+
+
+def make_topo_sortkey(model):
+    """
+    Returns a function suitable for use as a ``key`` kwarg to a
+    standard Python sorting call.  This key function will expect a
+    single class mapper and return a sequence number associated with
+    that model.  The sequence is determined by SQLAlchemy's
+    topological table sorting.
+
+    :param model: Usually the :term:`app model`, but can be any module
+       containing model classes.
+    """
+    metadata = model.Base.metadata
+    tables = dict([(table.name, i)
+                   for i, table in enumerate(metadata.sorted_tables, 1)])
+
+    def sortkey(name):
+        cls = getattr(model, name)
+        mapper = orm.class_mapper(cls)
+        return tuple(tables[t.name] for t in mapper.tables)
+
+    return sortkey

tests/db/test_util.py

@@ -11,6 +11,7 @@ try:
     from wuttjamaican.db import util as mod
     from wuttjamaican.db.model.base import Setting
     from wuttjamaican.util import make_true_uuid
+    from wuttjamaican.testing import DataTestCase
 except ImportError:
     pass
 else:
@@ -124,3 +125,17 @@ else:
             self.assertIsInstance(column, sa.Column)
             self.assertIsInstance(column.type, sa.String)
             self.assertEqual(column.type.length, 32)
+
+
+    class TestMakeTopoSortkey(DataTestCase):
+
+        def test_basic(self):
+            model = self.app.model
+            sortkey = mod.make_topo_sortkey(model)
+            original = ['User', 'Person', 'UserRole', 'Role']
+
+            # models are sorted so dependants come later
+            result = sorted(original, key=sortkey)
+            self.assertTrue(result.index('Role') < result.index('UserRole'))
+            self.assertTrue(result.index('User') < result.index('UserRole'))
+            self.assertTrue(result.index('Person') < result.index('User'))